1. Field of the Invention
This invention pertains to mobile telecommunications, and particularly to quality based power control in a code division multiple access (CDMA) mobile telecommunications system.
2. Related Art and Other Considerations
In general, a mobile telecommunications system or network includes numerous mobile stations, e.g., mobile telephones, which communicate over respective radio links or channels to a base station. The base station, which may include a base station controller, is typically connected by physical lines to a mobile switching station. The mobile switching station is often connected by physical lines to many base stations, as well as to other mobile switching stations. At least one, and usually several, mobile switching stations are connected to a gateway. The gateway interfaces the mobile communications system with a land-line (wired) communications system. Thus, when a non-mobile subscriber places a call to a mobile station, the call is directed to the gateway, and from the gateway through one or more mobile switching stations to the base station(s) which serve(s) the called mobile station. The base station employs a transceiver (e.g., transmitter and receiver) to page the called base station, ultimately resulting in the establishment of a connection when the mobile station responds.
In a code division multiple access (CDMA) mobile telecommunications system, the information transmitted between a base station and a particular mobile station is modulated by a mathematical code (such as spreading code) to distinguish it from information for other mobile stations which are utilizing the same radio frequency. Thus, in CDMA, the individual radio links are discriminated on the basis of codes. Various aspects of CDMA are set forth in Garg, Vijay K. et al., Applications of CDMA in Wireless/Personal Communications, Prentice Hall (1997). Typically the same base band signal with suitable spreading is sent from several base stations with overlapping coverage. The mobile terminal can thus receive and use signals from several base stations simultaneously.
In general, a CDMA base station attempts to maintain the same received power level in communications with each mobile station that it currently serves. To this end, a base station measures the received signal from each of the mobile stations in order to determine a parameter known as the Signal to Interference and Noise Ratio (SINR). For each mobile station, the SINR is compared with a target value SINR. If the SINR measured for a particular mobile station is less than the target value SINR, the base station commands the mobile station to increase its power in order that a stronger signal can be received at the base station. On the other hand, if the SINR determined for the particular station is greater than the target value SINR, the base station requests the mobile station to decrease its power. Thus, a power control loop is established between the base station and the mobile station, with an uplink aspect of the power control loop involving the mobile station transmitting to the base station and with the base station, upon receiving the signal from the mobile station and comparing the SINR for the station with the target SINR, providing power control commands to the mobile station as a downlink aspect of the power control loop.
As it turns out, the target SINR must be continually updated for numerous reasons, e.g., increase and decrease of the number of mobile stations served by the base station. The updating of the target SINR typically is part of another control loop, in particular a quality control loop between the base station and the mobile switching center. In an uplink aspect of the quality control loop, the base station provides the mobile switching center with an indication of the quality of connection for each of the mobile stations currently served. The mobile switching center uses such quality indication in order to calculate or otherwise determine the updated SINR target value. The updated SINR target value is then transmitted to the base station on a downlink aspect of the quality control loop.
One example of a control technique such as that described above is set forth in Pat. No. 5,623,484 to Muszynski, which is incorporated herein by reference. In order to transmit information (including a signal to interference and noise ratio setpoint [SINR])) between the base station and mobile switching center, Muszynski employs a common channel signaling technique known as Signaling System No. 7.
In common channel signaling, the signaling for a number of user channels/connections is carried in any number of signaling channels/connections. Moreover, there is no static relation between signaling channels/connections and user data.
In CDMA systems, the SINR target values must be updated frequently. When common channel signaling is employed as in Muszynski for updating a SINR target value, the load on the link or line connecting the mobile switching center with the base station is greatly increased due to the updates. For example, sending one common channel signaling message of ten to fifteen octets at a rate of ten to twenty times per second generates an additional signaling load of between one thousand and fifteen hundred bits per second per mobile station.
What is needed, therefore, and an object of the present invention, is a simple technique for maintaining quality-based power control in a CDMA mobile telecommunications system.